Re: How would Sarek explain Spock 'Prime' to the other Vulcan survivor

Old Spock has no reason to reveal his presence or any of the events in which he was involved.

The man arguably has certain knowledge of the future which would be enormously valuable, if one continues the pretense that there is a "Prime Universe" from which the new Trek continuity has diverged. He knows of the existence of species that the Federation does not yet, celestial events and phenomena ("there's a big probe that's going to want to talk to your humpback whales, it'll get here in about a quarter century and you might want to get working on that") and so on. If other people become aware of him, he'll quickly lose control of his own destiny.